The field of nonlinear optics has benefited from advances in nonlinear optical materials. Nonlinear frequency mixing devices pass input light through such nonlinear materials to drive nonlinear interactions between waves and generate output light. The nonlinear mixing mechanisms generating the output light typically involve one or more of the following: harmonic and sub harmonic generation, second harmonic generation, sum frequency generation, difference frequency generation, optical parametric generation and amplification and other three- or four-wave mixing processes. Cascaded processes, which occur simultaneously or in succession are also possible.
To achieve significant levels of frequency conversion the interacting waves have to remain in phase. Therefore, a phase-matching strategy has to be implemented in most of these devices. There are several phase-matching strategies. The most popular ones include type II phase matching using birefringence of the nonlinear optical material to prevent phase slip and quasi-phase-matching using a periodic variation in the nonlinear coefficient d to adjust for phase slip. Periodic variation of the nonlinear coefficient d is implemented, e.g., by a quasi-phase-matching (QPM) grating in which the value of the nonlinear coefficient d is reversed after 180° phase slip between the interacting waves.
In many nonlinear devices the interacting light waves are difficult to separate, or even distinguish, in space at the device output. In many cases this is because the interacting light waves propagate along the same optical path. This problem can be solved in some of these cases by using interacting waves with orthogonal polarizations in a type II phase-matching arrangement. However, this approach generally precludes the use of the largest nonlinear coefficient d. In addition, this approach is incompatible with some well-developed nonlinear material systems that integrate waveguides into the nonlinear material to better guide the interacting waves and increase conversion efficiency. Specifically, this approach cannot be used in proton-exchanged waveguides in lithium niobate (LiNbO3), which guide a single polarization of light.
In some cases wavelength selective filters can be used to separate the interacting waves. Unfortunately, this approach is cumbersome and inefficient in many cases. Furthermore, if one of the frequencies of the output light is the same as one of the input light frequencies, then this approach cannot be used. In nonlinear frequency mixing devices using waveguides with quasi-phase-matched (QPM) gratings, interferometer structures can separate mixed output from pump light and signal input light. The use of such interferometers renders wavelength selective filters unnecessary. Such devices are called optical-frequency balanced mixers and the reader is referred to Jonathan R. Kurz, et al., “Optical-frequency Balanced Mixer”, Optics Letters, Vol. 26, No. 16, August 2001, pp. 1283-1285 for further information. Although these devices can achieve good separation, further improvements in isolation and more efficient integration with waveguiding structures are desired.
Many prior art nonlinear frequency mixing devices use periodically poled gratings or poled structures in waveguides. The method of periodically poling such devices and controlling their poling is described in the prior art, e.g., in U.S. Pat. Nos. 5,630,004; 6,393,172 and Reissue Pat. No. 37,809. In addition, a method for controllable optical power splitting is addressed, e.g., in U.S. Pat. No. 5,488,681. These methods utilize poled structures (including asymmetric poled structures combined with waveguides) and applied electric fields. However, these references do not address nonlinear optical frequency mixing interactions between purely optical fields, and do not solve the problems of distinguishability and spatial separation in these cases.
The dependence of the propagation of the interacting waves on the characteristics of the waveguide has been recognized. For example, U.S. Pat. Nos. 5,872,884 and 5,991,490 teach an optical waveguide conversion device in which the refractive index and a thickness of the cladding layer are determined so as to satisfy a guiding condition for the light beam having a wavelength λ2 and a cutoff condition for a light beam having a wavelength λ1. While these designs teach that waveguides can be optimized for nonlinear mixing by tailoring the mode profiles, they do not address interactions between higher order modes and do not provide for easily separating and/or distinguishing them.
Additional teaching on variations in the core and cladding regions to alter the propagation properties of the interacting waves are discussed in U.S. Pat. No. 4,763,019 and the relationship of the propagation characteristics on temperature is further discussed in U.S. Pat. No. 5,546,220. Again, these references teach ways of manipulating the waveguide structure or temperature to facilitate nonlinear mixing between various modes, but do not address the problems of efficient nonlinear mixing between arbitrary odd, even, or asymmetric waveguide modes and their spatial separation.
In fact, engineering of the depth dependence of the nonlinear coefficient d has improved mode overlap efficiency in thin-film waveguides using higher-order modes. For some information on engineering the depth dependence the reader is referred to H. Ito, and H. Inaba, “Efficient Phase-Matched Second-Harmonic Generation Method in Four-Layered Optical-Waveguide Structure,” Optics Letters, 2, pp. 139-141 (1978). Engineering of the depth dependence of nonlinear coefficient d has also improved mode overlap efficiency in QPM channel waveguides with non-uniform domain inversion, as documented by M. L. Bortz, S. J. Field, M. M. Fejer, D. Nam, R. Waarts, and D. Welch, “Noncritical Quasi-Phase-Matched Second Harmonic Generation in an Annealed Proton-Exchanged LiNbO3 Waveguide,” IEEE Journal of Quantum Electronics, 30, Dec. 1994, pp. 2953-2960. It should also be noted, that in the early development of guided nonlinear optics, the phase-matching requirement often led to the use of higher-order mode interactions as discussed, e.g., by G. I. Stegeman and C. T. Seaton, “Nonlinear integrated optics,” Journal of Applied Physics, December 1985, Vol. 58, pp. R57-R77.
Unfortunately, none of the above apparatus or methods provide for simple, efficient, and robust spatial separation of the interacting waves in a nonlinear frequency mixer using QPM. What is needed is a simple and robust method of spatially separating the interacting waves that permits frequency mixing between multiple waves at the same or nearly the same wavelength. In addition, the method should allow for spectral inversion without wavelength offset and allow for simultaneous bi-directional wavelength conversion (so-called wavelength swapping).